This invention was developed with particular respect to gas turbine engine shafts and will be so described. The invention, however, is not limited to turbine engine shafts.
As commonly constructed, a gas turbine engine includes a hollow cylindrical case within which are mounted rows of stationary vanes, and a rotating shaft located axially within the hollow case upon which are mounted disks on whose circumferences are mounted a plurality of blades. Alternately arranged rows of moving blades and stationary vanes compress air and subsequent blade-vane combinations absorb energy produced by burning fuel with previously compressed air. Critical to the efficiency of such engines is the maintenance of minimum clearances between moving and stationary parts. The turbine shaft mounts the disks and blades for rotation and transmits power from the turbine section to the compressor section of the engine. Successful, efficient operation requires accurate location of the blades relative to the case. It is of the utmost importance that the turbine shaft be stiff and free from deflection and vibration (some vibration and deflection is unavoidable but the amount should be minimized). The stresses which produce deflection and vibration result from the engine operation and from externally applied loads resulting from aircraft motion.
Conventionally produced turbine shafts are fabricated from alloy steel and are hollow to derive the maximum specific stiffness.
The deflection under load of articles such as turbine shafts is inversely proportional to the modulus of elasticity, Young's modulus. Consequently, it is desirable to employ a shaft material having the highest possible modulus of elasticity to minimize deflections.
Metallic materials generally have a crystalline form, that is to say, individual atoms of the material have a predictable relationship to their neighboring atoms which extends in a repetitive fashion throughout a particular crystal or grain. The properties of such crystals vary significantly with orientation.
Most metallic articles contain many thousands of individual crystals or grains and the properties of such an article in a particular direction are reflective of average orientation of the individual crystals which make up the article. If the grains or crystals have a random orientation then the article properties will be isotropic, equal in all directions. Although widely assumed, this is rarely the case since most casting, deformation, and recrystallization processes produce a preferred crystal orientation or texture.
Textures have been extensively studied and practical use is made of textured materials, especially in the area of magnetic materials.
Crystals contain planes of atoms having particular spacings. These planes are identified by Miller indices of the form (111), (110), (100) etc. x-ray measurements can be made and texture intensities can be characterized as 1X, 5X random etc, with 5X random indicating a more intense texture than (for example) 2X random.
Metals that have undergone extensive deformation often display a "fibrous" macrostructure, especially when etched. Such a structure results from the alignment of inclusions, grain boundaries and second phases, but has no direct correlation with the crystallographic texture of the material, and should not be confused with the present invention.
It is an object of this invention to describe processing sequences which, when applied to a certain class of materials, can increase the Young's modulus or modulus of elasticity in the axial direction by as much as 25%.
It is also an object of this invention to describe the resultant high stiffness shafts.